Fluorinated oligothiophene donors for high-performance nonfullerene small-molecule organic solar cells

Thiophene is widely used in organic photovoltaics due to its excellent charge transport properties, simple synthesis, and high stability. In this paper, two simple oligothiophenes, namely, DC5T and 2FDC5T with rhodanine-flanked structures were synthesized and used as donors in nonfullerene small-molecule organic solar cells (NSM-OSCs). Compared to DC5T, 2FDC5T with fluorine atoms substituted on the conjugated backbone exhibited a downshifted highest occupied molecular orbital (HOMO) energy level and blue-shifted light absorption. With IDIC-4F as a small-molecule acceptor, the 2FDC5T-based device exhibited much higher short-circuit current density and open-circuit voltage than the DC5T-based device. As a result, a promising power conversion efficiency (PCE) of 9.02% was achieved for the 2FDC5T/IDIC-4F device, which was 28% higher than that of the DC5T/IDIC-4F device (PCE = 7.03%). Further research indicated that fluorination on the oligothiophene donor assisted the optimization of the active layer morphology and the reduction in the trap-assisted recombination in the device, which could deeply explain the improvement in efficiency.